This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:    3GPP third generation partnership project    BS base station    BW bandwidth    CRS common reference signal    CSI channel state information    CQI channel quality indicator    DCI downlink control information    DL downlink (eNB towards UE)    DM-RS demodulation reference signal (also known as URS)    eNB E-UTRAN Node B (evolved Node B)    EPC evolved packet core    E-UTRAN evolved UTRAN (LTE)    FDMA frequency division multiple access    HSPA high speed packet access    IMTA international mobile telecommunications association    ITU-R international telecommunication union-radiocommunication sector    LTE long term evolution of UTRAN (E-UTRAN)    LTE-A LTE advanced    MAC medium access control (layer 2, L2)    MCS modulation coding scheme    MIB master information block    MIMO multiple input multiple output    MM/MME mobility management/mobility management entity    NodeB base station    OFDMA orthogonal frequency division multiple access    O&M operations and maintenance    PDCCH physical downlink control channel    PDCP packet data convergence protocol    PDSCH physical downlink shared channel    PHY physical (layer 1, L1)    PMI pre-coding matrix indicator    PRB physical resource block    RACH random access channel    RE resource element    Rel release    RI rank indicator    RLC radio link control    RRC radio resource control (layer 3, L3)    RRM radio resource management    RS reference signal    SGW serving gateway    SIB system information block    TM transmission mode    SC-FDMA single carrier, frequency division multiple access    UE user equipment, such as a mobile station, mobile node or mobile terminal    UL uplink (UE towards eNB)    UMTS universal mobile telecommunications system    UPE user plane entity    URS UE-specific reference signal    UTRAN universal terrestrial radio access network
One modem communication system is known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE or as E-UTRA). In this system the DL access technique is OFDMA, and the UL access technique is SC-FDMA.
One specification of interest is 3GPP TS 36.300, V8.11.0 (2009-12), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (EUTRAN); Overall description; Stage 2 (Release 8), incorporated by reference herein in its entirety. This system may be referred to for convenience as LTE Rel-8. In general, the set of specifications given generally as 3GPP TS 36.xyz (e.g., 36.211, 36.311, 36.312, etc.) may be seen as describing the Release 8 LTE system. More recently, Release 9 versions of at least some of these specifications have been published including 3GPP TS 36.300, V9.3.0 (2010-03).
FIG. 1A reproduces FIG. 4.1 of 3GPP TS 36.300 V8.11.0, and shows the overall architecture of the EUTRAN system (Rel-8). Reference can also be made to FIG. 1B. The E-UTRAN system includes eNBs, providing the E-UTRAN user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UEs. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an EPC, more specifically to a MME by means of a S1 MME interface and to a S-GW by means of a Si interface (MME/S-GW 4). The S1 interface supports a many-to-many relationship between MMEs/S-GWs/UPEs and eNBs.
The eNB hosts the following functions:    functions for RRM: RRC, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both UL and DL (scheduling);    IP header compression and encryption of the user data stream;    selection of a MME at UE attachment;    routing of User Plane data towards the EPC (MME/S-GW);    scheduling and transmission of paging messages (originated from the MME);    scheduling and transmission of broadcast information (originated from the MME or O&M); and    a measurement and measurement reporting configuration for mobility and scheduling.
Of particular interest herein are the further releases of 3GPP LTE (e.g., LTE Rel-10) targeted towards future IMTA systems, referred to herein for convenience simply as LTE-Advanced (LTE-A). Reference in this regard may be made to 3GPP TR 36.913, V9.0.0 (2009-12), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Requirements for Further Advancements for E-UTRA (LTE-Advanced) (Release 9). Reference can also be made to 3GPP TR 36.912 V9.3.0 (2010-06) Technical Report 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Feasibility study for Further Advancements for E-UTRA (LTE-Advanced) (Release 9).
A goal of LTE-A is to provide significantly enhanced services by means of higher data rates and lower latency with reduced cost. LTE-A is directed toward extending and optimizing the 3GPP LTE Rel-8 radio access technologies to provide higher data rates at lower cost. LTE-A will be a more optimized radio system fulfilling the ITU-R requirements for IMT-Advanced while keeping the backward compatibility with LTE Rel-8.
As is specified in 3GPP TR 36.913, LTE-A should operate in spectrum allocations of different sizes, including wider spectrum allocations than those of LTE Rel-8 (e.g., up to 100 MHz) to achieve the peak data rate of 100 Mbit/s for high mobility and 1 Gbit/s for low mobility. It has been agreed that carrier aggregation is to be considered for LTE-A in order to support bandwidths larger than 20 MHz. Carrier aggregation (CA), where two or more component carriers (CCs) are aggregated, is considered for LTE-A in order to support transmission bandwidths larger than 20 MHz. The carrier aggregation could be contiguous or non-contiguous. This technique, as a bandwidth extension, can provide significant gains in terms of peak data rate and cell throughput as compared to non-aggregated operation as in LTE Rel-8.
A terminal may simultaneously receive one or multiple component carriers depending on its capabilities. A LTE-A terminal with reception capability beyond 20 MHz can simultaneously receive transmissions on multiple component carriers. A LTE Rel-8 terminal can receive transmissions on a single component carrier only, provided that the structure of the component carrier follows the Rel-8 specifications. Moreover, it is required that LTE-A should be backwards compatible with Rel-8 LTE in the sense that a Rel-8 LTE terminal should be operable in the LTE-A system, and that a LTE-A terminal should be operable in a Rel-8 LTE system.
In the context of LTE-A DL MIMO enhancements it has been decided to introduce two types of downlink reference signals. The first is referred to as DM-RS. DM-RS is a precoded UE-specific reference signal used for data detection/demodulation for up to eight spatial layers. The second DL reference signal is referred to as CSI-RS. CSI-RS is a cell-specific reference signal used for CQI/PMI/RI determination and channel sounding. CSI-RS has a lower density in time/frequency as compared to, for example, the CRS that is specified for use in Rel-8.
Reference can be made to 3GPP TSG-RAN Working Group 1 Meeting #57bis, R1-092474, Los Angeles, USA, 29 Jun.-8 May, 2009, Agenda Item: 15.1, Source: ZTE, Title: “Performance Evaluation for the Impact of CSI RS on Re18 PDSCH”. In this document the impact of CSI-RS insertion on the LTE Rel-8 PDSCH is discussed. It is said that replacement of Rel-8 PDSCH RE for LTE-A CSI-RS transmission may harm the Rel-8 PDSCH performance because the legacy UE would treat the corresponding REs as data and include them into the PDSCH channel decoding This situation is said to be generally worse than decoding with an erasure. Based on simulation results it was concluded that when the CSI-RS transmission interval is shorter than 5 ms the performance impact to the Rel-8 PDSCH is obvious in the case of a high modulation order or coding rate. To keep good performance for a 2 ms or 5 ms interval the CSI-RS should have low frequency density, e.g., less than 6 RE. Other remedies include MCS adjustment for a Rel-8 PDSCH packet when a RB has CSI-RS inserted. It was also noted that a uniform distribution of CSI-RS causes a smaller Rel-8 PDSCH performance loss than continuous distribution.
It has been agreed in 3GPP RAN1 that the CSI-RS density is one RE per antenna port per PRB per subframe. Two examples of CSI-RS subframe patterns are shown in FIG. 1C. CSI-RS is not necessarily present in each DL subframe, and it can be configured with a duty cycle of, e.g., 2, 5 or 10 ms. Reference in this regard can be made to, for example, 3GPP TSG-RAN WG1 Meeting #61, R1-102956, Montreal, Canada, May 10-14, 2010, Agenda item: 6.3.2.1, Source: Nokia, Nokia Siemens Networks, Title: “Intra-cell CSI-RS design”.
During RAN1 #60bis it was agreed that rate matching is to be applied to the CSI-RS locations for the Rel-10 UE, and that RE mapping of the PDSCH of the serving cell avoids CSI-RS of the serving cell. Reference in this regard can be made to 3GPP TSG RAN WGI Meeting #61, R1-102601, Montreal, Canada, 10-14 May 2010, Agenda item 3, Title: Final Report of 3GPP TSG RAN WG1 #60bis v1.0.0, (Beijing, China, 12-16Apr. 2010), Source: MCC Support.